Hi Mattb (I tried to respond by PM but either your PM box is full or you have not yet qualified to use it).
This has been addressed in the recently constructed 'draft' ICAO Heliport Manual in guidance on the subject of elevating the OLS (the issue you are trying to solve) and the lack of a clearway - necessary in most manufacturer's Category A procedures.
The Manual is due to be published later this year but if you are interested in an early explanation and some diagrams that address the issues, email me (you'll have to work out how to do this - LinkeIn would be one way) and I will try to explain, and show you, how both elements can be achieved with the types for which variable TDP procedures have been provided.
Regards
Jim